Template Haskell does not support type families.

{-# LANGUAGE TemplateHaskell, TypeFamilies #-}

import Language.Haskell.TH

type family A a


$((\x -> x >>= (runIO . (print . ppr)) >> return []) $
 [d|type instance A (Maybe Int) = Int|])

GHCi load result:

[1 of 1] Compiling Main             ( a.hs, interpreted )
type A = GHC.Types.Int
Ok, modules loaded: Main.

TySynD has list of Names as an argument list whereas type families header should have a list of Types.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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